1. Field of the Invention
This invention relates generally to a polyaxial screw and coupling apparatus for use with orthopedic fixation systems. More particularly, the present invention relates to a screw for insertion into spinal bone, and a coupling element polyaxially mounted thereto for coupling the screw to an orthopedic implantation structure, such as a rod, therein enhancing the efficacy of the implant assembly by providing freedom of angulation among the rod, screw and coupling element.
2. Description of the Prior Art
The bones and connective tissue of an adult human spinal column consists of an upper portion having more than 20 discrete bones, and a lower portion which consists of the sacral bone and the coccygeal bodies. The bones of the upper portion are generally similar in shape, as will be more fully described hereinbelow with respect to FIGS. 1 and 2. Despite their similar shape, however, they do vary substantially in size in accordance with their individual position along the column and are, therefore, anatomically categorized as being members of one of three classifications: cervical, thoracic, or lumbar. The cervical portion, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the 5 lumbar vertebrae.
These bones of the upper portion vary in size, but are each similarly coupled to the next by a tri-joint complex. The tri-joint complex consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. Referring now to FIGS. 1 and 2, top and side views of a typical vertebral body of the upper portion of the spinal column is shown. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 has three large protrusions, two of these extend laterally from the shell and are referred to as the transverse process 14. The third extends back and down from the lamina and is called the spinous process 16. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. Pedicles 24 are bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12 and posterior elements 14,16.
The lower portion of the spinal column, which extends into the hip region is primarily comprised of the sacral bone. This bone is unlike the other bones of the spinal column, in both shape and size. In fact, at birth humans have five distinct sacral bones which begin to fuse together during childhood, and by adulthood have fully combined. FIGS. 3 and 4 show side and perspective views of a sacral bone connected to the lower lumbar vertebrae.
From the side, the sacral body 50 appears horn-shaped, having an anterior 52 and a posterior side 54. The perspective view, however, demonstrates the sacral bone to be more plate-shaped, having a thin, curvate profile. The posterior side 54 includes the sacral foramena 56 which are, in fact, the fused former lamina portions of the original articulated sacral bones. Beneath the sacral foramena 56, the sacral roots of the spinal cord (not shown) are housed. The anterior 52 portion of the sacral body is, therefore, understood to be the fused former vertebral bodies of the originally articulated sacral bones. Extending laterally from the sacral plate are two, opposing, sacro ala 58, which are related to the transverse processes of the upper vertebrae. The sacro ala mate with the hip bones at the sacro-iliac joint 60. The sacro-iliac joint provides stabilization and support for the base of the spinal column, affording little if any rotational movement in adults. The top of the posterior portion of the sacral bone, the sacral foramena 56, couple with the lowest lumbar vertebra 62.
In its entirety, the spinal column is highly complex in that it houses and protects critical elements of the nervous system which have innumerable peripheral nerves and arterial and venous bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. The present invention relates to spinal fixation devices for immobilizing and altering the alignment of the spine over a large number, for example more than three or four, vertebra by means of affixing at least one elongate rod to the sequence of selected bones.
Such "rod assemblies" generally comprise a plurality of screws which are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws are provided with coupling elements, for receiving an elongate rod therethrough. The rod extends along the axis of the spine, coupling to the plurality of screws via their coupling elements. The aligning influence of the rod forces the spine to which it is affixed, to conform to a more proper shape.
It has been identified, however, that a considerable difficulty is associated with inserting screws along a misaligned curvature and simultaneously exactly positioning the coupling elements such that the receiving loci thereof are aligned so that the rod can be passed therethrough without distorting the screws. Attempts at achieving proper alignment with fixed screws is understood to require considerably longer operating time, which is known to increase the incidence of complications associated with surgery. Often such alignments, with such fixed axes devices could not be achieved, and the entire instrumentationing effort would end unsuccessfully.
In addition, for many patients specific pathology it is desirable that the rod extend down into and beyond the lumbar portion of the spine, and for the end of the rod to be coupled to the sacral bone. Providing such an end to the assembly in the sacral bone has been understandably suggested inasmuch as it provides superior support to the full extent of the assembly. The most suitable position for the insertion of the screws into the sacral body may not, however, conform to the direction extent of the rod as it is affixed to the entirety of the assembly. Misalignment of the rod with respect to the screw and the coupling element is often a source of considerable disadvantage for the surgeon, often requiring considerable efforts to be expended bending and aligning the rod with the receiving locus of the coupling element. These additional efforts are a considerable difficulty associated with the proper and expeditious affixation, and over the long term, the offset of the rod can have a deleterious effect on the overall performance of the entire implantation assembly.
The art contains attempts at providing instrumentation, for example as set forth in U.S. Pat. No. 5,207,678 to Harms et al., which permit a freedom with respect to angulation of the screw and the coupling element. These teachings, however, have generally been complex, and inadequately reliable with respect to durability. Foe example, U.S. Pat. No. 5,207,678, teaches a pedicle screw having a "spherical segment-shaped head". The screw is inserted through a first receiving element, into the pedicle, such that the spherical head is disposed in the curvate base of the first receiving element. Once properly angled, a second inner receiving element is positioned within the first receiving element. The rod, which is necessarily threaded is then positioned within the two nested receiving elements. Nuts on the rod, positioned on opposing sides of the receiving element are then simultaneously tightened relative to one another in order to lock the rod to the receiving elements. The locking of the nuts to the receiving element is further intended to lock the screw in the proper angulation with respect to the receiving elements and the rod.
The considerable drawbacks associated with such a system include the necessity to prepare the threaded rod with a pair of nuts, properly positioned along the extent of the rod, which must be individually tightened or moved during the implantation procedure. Manipulation of the nuts is tedious in the ideal preparation, and often requires removal and considerable adjustment of the nut positions even after a portion of the rod has been implanted.
An additional difficulty with the above described Harms et al. system is that it requires a plurality of elements, such as nesting receiving elements and multiple nuts, which negate the simplicity and efficiency of implantation which is desired in the surgical field.
It is, therefore, the principal object of the present invention to provide a pedicle screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception.
In addition, it is an object of the present invention to provide such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation.
Accordingly it is also an object of the present invention to provide an assembly which is reliable, durable, and provides long term fixation support.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.